Aust colluded with oil firms: East Timor

Xanana Gusmao has accused a United Nations commission of a lack of impartiality.

East Timor has accused the UN of bias and Australia of collusion over the processing of oil and gas from the Greater Sunrise reserve, as it prepares to sign a deal to end a long-running maritime boundary dispute.

Foreign Minister Julie Bishop and East Timor's chief negotiator Xanana Gusmao are expected to sign a treaty at the United Nations in New York at 9am eastern Australian time on Wednesday.

East Timor has succeeded in its bid to push the maritime border out halfway between the two countries, placing much of the oil and gas fields in its territory.

The reserve is estimated to be worth upwards of $56 billion.

Questions over where the gas will be processed are yet to be settled.

In a letter to the UN Conciliation Commission leaked to the ABC, Mr Gusmao accused Australia of colluding with oil companies to have resources piped to Darwin.

"The commission instead opted for the easiest way out, which is a shame as in my perception it reveals a lack of impartiality on your behalf," Mr Gusmao wrote.

"Civil society from both countries could also potentially perceive this as a form of collusion between the government of Australia, Darwin LNG partners and/or the Sunrise (joint venture), with the support of the commission."

Australia has pushed for a 80/20 split of the reserve so long as Darwin processes the resources.

East Timor wants a 70/30 split but wants processing conducted on its soil.